The statements in this section merely provide background information related to the present disclosure and may or may not constitute prior art.
In automatic shifting power transmissions, gear ratio changes are effected by selectively connecting members of planetary gear sets. This is accomplished by selectively engaging torque-transmitting devices such as brakes or clutches. For each gear ratio change, there is a corresponding sequence of off-going clutch disengagement and on-coming clutch engagement. By providing a regulated fluid pressure gain to the clutches, the smooth engagement and disengagement of the clutches may be accomplished.
Additionally, a majority of automatic shifting power transmissions employ a hydrodynamic fluid drive, such as a torque converter or a fluid coupling, between the power source (engine) and a multi-speed gear configuration, such as the planetary gear arrangement discussed above. This hydrodynamic fluid drive will allow the vehicle to come to rest without stalling the engine and will provide a measure of isolation preventing the torsional vibrations, caused by the firing events of the engine, from being transmitted though the powertrain.
As is well known, the hydrodynamic fluid drive is a slipping drive that has a high efficiency loss at vehicle launch. This loss decreases, but is still present, as the hydrodynamic fluid drive approaches a 1 to 1 speed ratio at high speed and low torque.
The stall speed of the hydrodynamic fluid drive is an important consideration for proper vehicle launch. The stall speed is the speed at which the hydrodynamic fluid drive will hold the engine speed and not allow further gain. The stall speed is usually chosen based on engine torque characteristic, vehicle weight, vehicle duty cycle, etc. A properly selected stall speed will allow the engine to spin to the peak torque range to affect a strong vehicle launch.
However, once a hydrodynamic fluid drive is selected with a specific stall speed that stall speed cannot be adjusted regardless of changing conditions or circumstances. Therefore, it is desired to be able to alter the effective stall speed of the hydrodynamic fluid drive. Under certain circumstances a higher stall speed is desired to improve launch performance of the transmission. Under other circumstances, a lower stall speed is beneficial by providing sufficient launch performance while improving efficiency or other performance parameters. Accordingly, there is room in the art for controlling a powertrain to modify the stall characteristics of a fluid coupling device to increase efficiency.